Separation of very close boiling components in petroleum streams such as cycloalkanes and alkanes by conventional distillation is both impractical and uneconomical. More specific examples of difficult separations are the separation of cyclohexane and 2,4 dimethylpentane, or cyclopentane and 2,2 dimethylbutane. One of the alternative methods of separating close boiling components is extractive distillation (ED). In an ED column, a polar nonvolatile solvent is introduced into the column near the top to preferentially associate with the more polar components in the feed mixture, so that the relative volatility between the close boiling components can be significantly increased, making the separation possible. The basic principles, design, and operation of ED processes have been thoroughly discussed in the literature: Atkins, G. T. and C. M. Boyer, Chem. Eng. Prog., 45, 553 (1949); Chambers, J. M., Chem. Eng. Prog., 47, 555 (1951); Hackmuth, K. H., Chem. Eng. Prog., 48, 617 (1952); Butler, R. M. and J. A. Bichard, U.S. Pat. No. 3,114,783 (1963); and Perry's Chemical Engineers' Handbook, 6.sup.th Edition, McGraw-Hill Book Company, 1984, pp. 13-53 to 13-57, the disclosures of which are herein incorporated by reference.
The separation of cycloalkanes (cycloparaffins), in particular cyclohexane or cyclopentane, from close-boiling alkanes (isoparaffins) by extractive distillation is known and has been described in the patent literature, such as in U.S. Pat. Nos. 2,508,723; 2,771,494; 2,846,485; 2,891,894; 3,034,969; 4,053,369; 4,921,581; 4,944,849; 4,948,470; 4,948,472; 4,954,224; 4,955,468; 5,032,232; 5,055,162; and 5,069,756, the disclosures of which are herein incorporated by reference. Some significant improvements have been made recently in formulating mixed solvents in order to effectively separate cycloalkanes from alkanes, in particular cyclohexane or cyclopentane from close-boiling isoparaffins. However, it is highly desirable to develop further improved extractive distillation processes for producing cyclohexane or cyclopentane of high purity from mixtures of isoparaffins, employing single extractive solvents as well as more selective mixed solvents.